The invention relates generally to a method for generating an anti-reflection coating for an optical gain element, and in particular to determining improved coating parameters for an anti-reflection coating for a laser diode.
Anti-reflection coatings are known to reduce the reflectivity of various optical components, such as lenses and windows, thereby increasing the transmittance of the optical components. Optical gain elements, such as laser diodes, can also be coated to improve their performance. Unfortunately, optical gain elements provide significant challenges to the processes of designing and fabricating anti-reflection coatings.
Many optical gain elements include a waveguide or active region in which optical energy is generated and/or amplified. Modeling these elements requires a knowledge of the waveguide structure (e.g., refractive index and dimensional parameters) and the distribution of optical energy in the active region. The effective refractive index and optical mode confinement can vary as operating parameters, such as the current or voltage applied to the element, are changed. Thus, manufacturers often have difficulties designing and fabricating laser diodes with highly dependable performance specifications.
The optical coating process typically introduces additional performance uncertainties to the performance characteristics of the anti-reflection coated optical gain element. Due to variations in process parameters between coating facilities, anti-reflection coatings fabricated according to a specific design in one facility generally have different performance characteristics than anti-reflection coatings fabricated to the same design in other facilities. In addition, variations in the supplied materials used in the coatings (e.g., refractive index variations) and variations in the characteristics of the optical gain element prior to coating (e.g., variations in output beam characteristics) also result in variations in the performance of the coated elements. Consequently, the performance of a system utilizing anti-reflection coated optical gain elements is generally dependent on the source of the optical gain elements, the source of the coating materials and the particular coating facility used to fabricate the coatings.
The invention relates to a method for generating an anti-reflection coating on an optical gain element. According to one embodiment, the method includes determining coating parameters for an anti-reflection coating based on known characteristics of the optical gain element and fabricating the anti-reflection coating according to the coating parameters on an optical gain element. The performance of the coating on a first optical gain element is measured and one of the coating parameters is modified in response to the measurement. In one embodiment, measuring the performance of the coating on the first optical gain element includes measuring the lasing threshold current of the first optical gain element. In another embodiment, modifying one of the coating parameters includes changing the thickness of a coating layer. In another embodiment, modifying one of the coating parameters includes changing the refractive index of a coating layer. An anti-reflection coating based on the modified design is fabricated on a second optical gain element. The performance of the anti-reflection coating on the second optical gain element is measured. Improved coating parameters are determined in response to the measurements of the anti-reflection coatings on the first and second optical gain elements, respectively. The coating parameters may include the identities of coating materials, refractive indices of the coating materials and/or coating layer thicknesses.
In a further embodiment, the steps of fabricating the anti-reflection coating on the first optical gain element and measuring its performance includes fabricating the anti-reflection coating on the first optical gain element, measuring the performance of the anti-reflection coating on the first optical gain element, modifying a second coating parameter based on the measured performance of the anti-reflection coating on the first optical gain element, fabricating a modified anti-reflection coating on the first optical gain element based on the modified second coating parameter and measuring a performance of the modified anti-reflection coating on the first optical gain element.
The invention also relates to an optical gain element having an anti-reflection coating made by the above method. In one embodiment, the optical gain element is a laser diode.